1. Field of the Invention
The present invention relates to a magnetic recording method for heat-assisted recording of information on a magnetic recording medium. The present invention also relates to a test record reproduction method for determining the recording conditions that will be employed in the heat-assisted magnetic recording method.
2. Description of the Related Art
Magnetic recording media (magnetic disks) are known as recording media for constituting recording devices such as hard disks. The increase in the information processing volume in computer systems creates a growing demand for magnetic disks with increased recording density.
When information is recorded on a magnetic disk, a magnetic head for recording is disposed close to the recording surface (composed of a recording magnetic film) of the magnetic disk, and a recording magnetic field with intensity higher than the coercive force of the recording magnetic film is applied to the recording magnetic film with the magnetic head. When the direction of the recording magnetic field from the magnetic head is successively inverted, while the magnetic head is being moved with respect to the magnetic disk, a plurality of magnetic domains (recording marks) with successively inverted magnetization direction are formed in a row in the circumferential direction of the magnetic disk or in the track extension direction in the recording magnetic film. At this time, the recording marks with prescribed individual length are formed by controlling the timing at which the direction of the magnetic recording field is inverted. The prescribed signals or information are thus recorded as changes in the magnetization direction in the recording magnetic film.
In the technological field of magnetic disks, it is well known that the larger is the coercive force of the recording magnetic film, the higher is the thermal stability of magnetic domains formed in the recording magnetic film and the easier is the formation of stable magnetic domains of a very small size or width. The smaller are the smallest magnetic domains that can be formed with good stability in the recording magnetic film, the higher is the recording density that can be obtained in the magnetic disk.
In information recording on magnetic disks, as mentioned hereinabove, recording marks cannot be adequately formed unless the recording magnetic field applied to the recording magnetic film is higher than the coercive force thereof. For this reason, the intensity of the recording magnetic field that has to be applied by the magnetic head apparently has to increase with the increase in the coercive force that is set for the recording magnetic film. However, the intensity of the recording magnetic field that can be applied by the magnetic head is limited, for example, from the standpoint of magnetic head structure or power consumption.
Accordingly, the so-called heat-assisted magnetic recording method is sometimes used for information recording on the magnetic disks. When information recording on magnetic disks is implemented by employing the heat-assisted magnetic recording method, first, the temperature of the recording magnetic film of the magnetic disk is locally and successively raised by conducting laser beam illumination from the prescribed optical head disposed in the vicinity of the recording surface of the rotating magnetic disk. In the area with the raised temperature in the recording magnetic film, the coercive force decreases with respect to that in the surrounding areas where the temperature has not been increased. Then, a recording magnetic field that is stronger than the coercive force in the area of the recording magnetic field with the increased temperature is applied to the area with the increased temperature by the magnetic head disposed in the vicinity of the recording surface of the magnetic disk, and part of the area with increased temperature is magnetized in the prescribed direction. This magnetization is fixed in the process of cooling the magnetized zone. With the heat-assisted magnetic recording method, recording marks magnetized in the prescribed direction are thus formed. If the heat-assisted magnetic recording method is used, information recording is implemented by applying the recording magnetic field to the zones in which the coercive force was decreased by heating. Therefore, even when the coercive force of a recording magnetic film at a normal temperature during information storage or information reproduction is set at a high level, it is not necessary to increase excessively the intensity of the recording magnetic field that has to be applied with the magnetic head. Such a heat-assisted magnetic recording method is described, for example, in Japanese Patent Applications Laid-open No. H6-243527 and 2003-157502.
With the conventional heat-assisted magnetic recording method, the rotation speed of magnetic disk is set to a constant value and the power of the illuminated laser beam is also set to a constant value. Because the rotation speed of the magnetic disk is constant, the linear speed of each zone in the recording surface of the magnetic disk during information recording changes according to the distance from the rotation center.
More specifically, the linear speed is lower in zones closer to the rotation center. The laser beam illumination interval (heating interval) of the region is longer in the zones with a low linear speed (zones close to the rotation center). Therefore, the energy of temperature increase that is received per unit time is larger in the zones with a low linear speed. Moreover, the cooling efficiency resulting from the rotation of the magnetic disk is lower in zones with a low linear speed.
Therefore, with the conventional heat-assisted magnetic recording method, the maximum attained temperature (the highest temperature in the temperature increase area attained by laser beam illumination) is higher and the range of temperature increase above the prescribed temperature is wider under laser beam illumination in the zones of the recording surface where the linear speed is low, that is, the zones close to the rotation center. Therefore, with the conventional heat-assisted magnetic recording method, the maximum attained temperature in the temperature increase area during the formation of recording marks in the zones on the outermost periphery (that is, zones with the lowest heating efficiency), which is the farthest from the rotation center, on the recording surface is correspondingly different from the maximum attained temperature in the temperature increase area during the formation of recording marks in the zones on the innermost periphery (that is, zones with the highest heating efficiency), which is the closest to the rotation center, on the recording surface.
Futher, the width of the temperature increase range above the prescribed temperature during the formation of the recording marks in the zones on the outermost periphery of the recording surface differs correspondingly from the width of the temperature increase range above the prescribed temperature during the formation of the recording marks in the zones on the innermost periphery. With such a conventional heat-assisted magnetic recording method, when the illumination laser power is set to an intensity at which recording marks can be formed in the zones on the outermost periphery of the recording surface, during information recording on the inner peripheral side of the recording surface, the illumination laser power becomes too high. As a result, in some cases the maximum attained temperature of the recording magnetic film increased by laser beam illumination becomes inappropriately high and the temperature increase range above the prescribed temperature becomes inappropriately wide.
In the heat-assisted magnetic recording method in which recording marks are successively formed along the tracks of the recording magnetic field by laser beam illumination of the recording magnetic field and application of the recording magnetic field thereto, if the maximum attained temperature is too high, the coercive force in the formation of the recording marks that were formed immediately before this event drops inappropriately, sometimes causing the recording demagnetization effect, that is, loss or degradation of the recording marks formed immediately before this event under the effect of the recording magnetic field. The recording demagnetization effect is undesirable because it decreases the SNR (Signal to Noise Ratio) in the reproduced signals during magnetic disk reproduction and impedes the increase in magnetic recording density of the magnetic disk. Furthermore, if the temperature increase range above the prescribed temperature is too wide, the recording mark width inappropriately increases, reaching the track adjacent to the track where the recording marks have been successively formed and sometimes causing the cross-write effect, that is, loss or degradation of recording marks on the adjacent tracks. The cross-write effect is undesirable from the standpoint of increasing the recording density of magnetic disk because it hinders the decrease in the pitch between the tracks. Thus, the conventional heat-assisted magnetic recording method in which the illumination laser power which is so set that the recording marks can be formed in the zones on the outermost periphery of the recording surface is employed during information recording on the inner peripheral side of the recording surface impedes the increase in recording density.